What's your system specs, OS (XP, 2000, etc.) anti-virus, etc.

Try installing in "Safe Mode":

 

Shut down computer> Restart, then almost right away before any screen comes up tap the F8 button. Some code will flash, then a short menu in a DOS-like screen will come up>Select "Safe Mode without networking, and hit Enter>The computer will boot into your desktop with a black background and the words "Safe Mode" in the corners of your screen>Try the installation.

 

If this doesn't work, and/or to get back into normal mode, just shutdown and reboot.

 

Also if you haven't done it already, make sure you have all the updates and patches from windows update, your anti-virus is updated and you've scanned, and any new drivers from Dell are downloaded.